Support HED (Hierarchical Event Descriptors) for epoching

Describe the new feature or enhancement

MNE-Python represents events by a set of discrete integer codes, and epoching functions mostly select epochs based on these codes.

I was interested in incorporating some epoching features based on queries of HED strings and was hoping to get some advice (and maybe a little help) on figuring out the best way to do that.

HED is incorporated as an event annotation mechanism in BIDS. We can easily produce a table of event number versus HED string annotations to start with. (Note these would be the original event positions not codes in a particular column, since HED annotations are usually assembled from information in several columns in a BIDS event file.) We can also select numbers based on a HED string query.

Describe your proposed implementation

I'd really appreciate some advice on where to start on this.

Describe possible alternatives

We could extend the Epoch class so that it would store the HED annotations as a function of sample number.

We could also do something with the Annotation class.

Perhaps better, we could write an alternative way of creating Epochs and as part of this attach a HED_assembled column to the Epochs metadata. We could then write a query interface.

Additional context

Note:
In a BIDS dataset the HED annotation for each event is assembled from a combination of the information in the events.tsv HED column and the information in the other columns based on any applicable sidecars. This information is combined during analysis to provide a single HED annotation of each event.

Start by reading the epoching functions and the Epoch and Annotation classes named in the issue, then trace how BIDS event HED annotations are assembled. The issue does not identify files or tests. Done requires a decided design and a query-based HED epoching workflow, but the exact interface and validation scope remain open.